This invention relates generally to a design of a binary position sensor, and more specifically, to an apparatus for determining the position of a laser beam at a resolution which is smaller than the laser beam size.
Traditionally, photo sensors are used for different purposes, such as measuring angle, distance, height, centering, surface uniformity and other parameters related to position sensing. One of the most used applications of photo sensors is monitoring the displacement of a path that a scanning beam covers in reference to a line to be scanned.
Different sensors use a grating with binary format apertures, as shown in FIG. 1, either on a rectangular plate 10 or a circular disk (not shown) for measuring parameters related to position sensing or for measuring the rotation angle or the number of rotations. The apertures 12 on the grating tracks t.sub.1, t.sub.2 and t.sub.3 provide a passage for the light and the nontransparent segments 14 (the areas between the apertures on each track) block the light from passing through. By placing a light source 16 on one side of the grating plate or the grating disk, the light emitted from the light source passes only through the apertures 12. On the opposite side of the grating plate or the grating disk, where there is an aperture 12 there is light and where there is nontransparent segment 14 there is darkness. The light and the darkness can be sensed by having one sensor for each grating track. The grating plate 10 moves in X--X direction. Therefore, the light sensors 18, 20 and 22 receive light or darkness depending on if they are positioned behind an aperture 12 or behind a nontransparent segment 14. In the example shown in FIG. 1, the light sensors 18 and 20 which receive the light generate an electric signal representing logic 1 and the sensor 22 which is in dark stays inactive representing logic 0. By reading out the outputs of the sensors with each sensor representing one binary bit, the binary position of the plate or the angle of rotation is determined.
The above apparatus does not have a high resolution. In order to improve the resolution, the number of the grating tracks should be increased and in each grating track the number of apertures should also be increased. The problem arises when the apertures and the nontransparent segments become smaller than the width of the light or the laser beam, shown in FIG. 2. In this case, if the peak N--N of the Gaussian distribution 25 of the light beam happens to be on a nontransparent segment 14, but the beam size extends over onto the adjacent aperture 12, instead of reading out a logic 0 for darkness 26, a logic 1 for light 28 passing through aperture 12 is read out. Therefore, increasing the number of grating tracks and the number of apertures is not an effective way of improving the resolution.